Good idea, I think it's called a completion tooltip?.. it auto-completes the @username when you start to type it and highlights the message you send to them (only they see it highlighted).

It would solve the issue of everyone's chat text being the same colour in the global lobby..

..It would be pretty cool if the highlighted text appeared in your game chat (as well as the lobby chat) because one of the curious features about the lobby system is our screennames remain in the lobby even after we have found a game, it's weird having players see you as available (because you're name is still showing in the lobby) when you're actually in a game.

I think players that are in a game should at least have an icon next to their screenname (in the lobby) so that players in the lobby know you're in-game.. this small feature would let players know you're not available for invites etc ..It can be distracting knowing players are possibly trying to talk to you in a lobby you're not present in.

One other useful icon to put beside screennames would be a volunteer icon, something that lets players know you're free and ready to ally.